A problem in the art of manufacturing and processing thermoplastic polymers and coating compositions is their instability upon extended exposure to ultraviolet (UV) light sources. It is well known that light, oxygen and heat cause degradation of polymers resulting in deterioration of mechanical and physical properties of the polymer. Coatings and plastics tend to demonstrate unwanted color changes and reduced mechanical strength upon exposure to UV radiation. To prevent or at least retard the damage caused by these factors, UV light absorbing compounds are added to the plastic as stabilizers.
Benzotriazoles have long been an important class of UV absorbers and have gained wide commercial importance and acceptance as UV stabilizers for many industrial applications. The prior art is replete with references to their manufacture and utility. For example:
U.S. Pat. No. 3,004,896 to Heller et al demonstrated that hydroxyphenyl benzotriazoles possess desirable overall absorption characteristics for application involving the protection of UV light absorbing polymer compositions since they absorb very strongly within the UV region but absorbed very little light as the wavelength approaches 400 nm, i.e., are colorless or non-actinic in the visible spectrum.
U.S. Pat. No. 4,275,004 to Winter et al discloses a high caustic coupling process for preparing o-nitroazobenzene intermediates useful in the synthesis of 2-aryl-2H-benzotriazole light absorbers.
There are a variety of considerations when choosing a suitable UV light stabilizer. Compatibility is one of them. Many stabilizers prove to be incompatible with the polymer or coating composition being stabilized. Another important consideration is the volatility of the stabilizer in the polymer and during its processing. The stabilizer must have sufficiently high molecular weight so that it remains in the polymer matrix for the life of the plastic or coating. Many polymers are processed at very high temperatures which causes the stabilizer to vaporize.
To improve polymer compatibility and reduce volatility, appropriate side chains have been added to the phenolic ring or the benzene ring of the benzotriazole. For example,
U.S. Pat. Nos. 4,226,763 and 4,278,589 to Dexter et al disclose 2-(2-hydroxyphenol)benzotriazoles substituted ortho and/or para to the hydroxyl group by cumyl or substituted cumyl groups.
U.S. Pat. No. 3,766,205 to Heller et al describes 2-(2-hydroxyphenol)benzotriazole light stabilizers having methyl and carbododecyloxyethyl groups ortho and para to the hydroxy group which are employed, for example, in polyamides and polyesters.
Japanese Patent Application Sho 54-95,233 describes 2-(2-hydroxyphenol)benzotriazole light stabilizers having tert.butyl groups ortho and para to the hydroxyl group which are employed together with co-stabilisers in materials for colour photography.
U.S. Pat. Nos. 4,853,471 and 5,032,498 to Rody et al disclose 2-(2-hydroxyphenol)benzotriazole UV absorbers having tert.butyl groups and —CH2CH2COOR acid or ester groups ortho and para respectively to the hydroxy group which are used in particular for stabilizing and improving lacquers and photographic material. The compounds can be either monomers or ester linked dimers of the substituted 2-(2-hydroxyphenol)benzotriazole.
U.S. Pat. Nos. 5,554,760 and 5,574,166 to Winter et al discloses a soluble, crystalline form of 2-(2-hydroxyphenol)benzotriazole which is substituted in the 3-position (ortho to the hydroxyl group) with an α-cumyl group and in the 5-position (para) usually with a mixture with various alkylated moieties as substituents or as single compounds substituted with one bulky, tert-alkyl group.
U.S. Pat. Nos. 5,977,219 and 6,166,218 to Ravichandran et al disclose 2-(2-hydroxyphenol)benzotriazole monomers and dimers which are substituted at the 5-position of the benzo ring by an electron withdrawing group. The compounds are claimed to exhibit enhanced durability and low loss rates when incorporated into automotive coatings particularly when the 3-position of the phenyl ring is also substituted by phenyl or phenylalkyl, e.g., 5-trifluoromethyl-2-(2-hydroxy-3-α-cumyl-5-tert-octylphenyl)-2H-benzotriazole.
U.S. Pat. No. 5, 240,975 to Winter et al discloses liquid substituted 2H-benzotriazole mixtures prepared by alkylation with concomitant dealkylation, fragmentation and realkylation of 2-(2-hydroxy-5-alkylphenyl)-2H-benzotriazoles or of 2-(2-hydroxy-3,5-dialkylphenyl)-2H-benzotriazoles with straight or branched alkenes of 8 to 30 carbon atoms in the presence of an acid catalyst at 100-200° C.
Since the types of UV light sensitive polymers and compositions employed in different applications is diverse there is not a single set of side chain substituents or “modifying groups” that will provide an optimal set of properties for these diverse chemical systems. Furthermore, in most cases the modifying groups do not add substantially to the UV absorption characteristics of the molecule, such that the additional molecular weight of the modifying group does not contribute to the light absorption properties of the UV light stabilizer. Consequently, the UV absorption properties of the modified stabilizer can actually be reduced because of a relative lowering in molar extinction coefficient.
One approach to improve the level and efficiency of UV light protection has involved the use of combinations of materials. For example:
U.S. Pat. No. 3,496,136 to Susi et al disclosed a combination of UV absorbers and substituted melamine derivatives as stabilizers for rigid thermoplastic resins such as PVC or polystyrene.
U.S. Pat. Nos. 4,413,933 and 4,426,471 to Berner discloses the use of various sterically hindered amine light stabilizers such as N-substituted 2,2,6,6-tetraalkylpiperadine compounds, their acid addition salts or complexes with metal compounds, optionally together with further stabilizers, especially substituted hydroxyphenyl benzotriazoles, for stabilizing acid catalyzed stoving lacquers.
Several studies have attempted to make UV absorbers more efficient and/or effective by introducing different substituents or by coupling known UV chromophores to the benzotriazole which could increase intrinsic absorption. For example:
U.S. Pat. No. 3,936,418 to Pond et al discloses benzotriazole ortho-ester ultraviolet light stabilizers and their use in organic compositions. Specifically, substituted 2-(2-hydroxyphenol)benzotriazole were reacted with aryl acid chlorides, e.g., benzoyl chloride or isophthaloyl chloride, to form an ortho ester, i.e., esterification of the 2-hydroxy group of the phenyl ring. The use of di- or tri-functional aryl carboxylic acids produced multimer esters of 2-(2-hydroxyphenol)benzotriazole. No data was disclosed on the UV absorption efficiency of the ortho esters or a comparison with other hydroxyphenyl benzotriazoles.
U.S. Pat. No. 5,362,881 to Leistner et al found that the volatility of benzotriazole and benzophenone stabilizers can be sharply reduced by coupling these different UV light absorbing chemical moieties utilizing a methylene bridge. The UV absorber obtained is composed of a discrete benzotriazole moiety and a discrete benzophenone moiety joined by a methylene group. The UV light absorption characteristics of the molecules were not reported.
As requirements for durability, effectiveness and use of certain solvents become ever more stringent and demanding, the search for still more stable, durable, more effective and efficient UV stabilizers continues. Furthermore, because of the diversity of polymer systems employed in plastics and coatings there is also a continuing need for versatile process chemistry that can provide a suite of ultra-violet light absorbing compounds which can be tailored to different polymer applications.
It has now been found that a range of ketones of 2-(2-hydroxyphenyl)benzotriazoles having an acyl group substituted ortho to the hydroxy group of the phenyl ring can be efficiently prepared. Surprisingly, the ketone substituents were found to increase UV absorption and did not lead to a decrease in absorption as has been found for bulky 3′ alkyl groups in 2-(3′-alkyl-2′hydroxyphenyl)benzotriazoles and the 2-hydroxyphenyl naphthotriazoles (e.g., U.S. Pat. No. 4,129,521 to A. Strobel). The UV absorbers of the present invention have structural elements contained in both 2-(2-hydroxyphenyl)benzotriazoles and hydroxybenzophenones. In particular, the adjacency of the hydroxyl and keto groups on the “central” phenyl ring provides a “hybrid” structure which allows for hydrogen bonding between these groups and is believed to lead to the increased UV absorption characteristics of the compounds which improves their efficiency and spectral coverage.
In some hybrid variants in which the ketone bridges a 2-(2-hydroxyphenyl)benzotriazole and a benzene ring the compounds have UV absorption characteristics of a benzotriazole and a benzophenone. That is, the middle hydroxyphenyl group is shared between the benzotriazole and the phenone.
As such the ketone substituents of the instant compounds not only influences physical properties such as melting point but also provides enhanced UV absorption efficiency, i.e., molar extinction coefficient and broader coverage across the UV A and B spectrum.